Would increasing the world map resolution allow for tighter tree placement? so that Forest look more like forest?
I'm not saying to increase the size of the world, just how detailed the world map can be.
There would be many advantages to doing this.
If not then
SUGGESTION: Forest and thickets mod.
This duplicates the ground model "above" the trees with a "leaf canopy" texture. Not a completely solid texture. it would have transparencies so the leaves look like leaves and not an upside down layer of grass.
This same method allows for "thickets" or chest height overgrowth.
If re-applied in another method it could allow for 'deep swamps', field grass, streams and rivers you have to traverse.